Portable computers are becoming more and more popular in the business world. In addition to being convenient, portable computing allows an organization to extend its production system further into the field. Whether a user is taking orders, doing field service, or delivering packages, the user is executing transactions against an organization's production system. Portable computers enable a user to perform these transactions on the spot and in a timely manner.
A user of a portable computer is, in effect, operating as a client to the organization's production system server. The client is, however, not usually connected to the server and must operate autonomously, such as by taking orders or querying the status of orders. To successfully perform these tasks, the client maintains a local replica of the server's data.
While multiple copies of data exist, these copies need to be kept consistent. While disconnected, applications at each client and at the server may modify their data. To achieve consistency, connections must be made and individual clients synchronized with the server. In this way, modifications made at one client are propagated to the server and eventually to each client as it connects.
The synchronization is difficult to achieve, even in situations where the server and all clients are running the same database management system software. Unfortunately, this is not generally the case. Clients are often personal computers running single-user database software whereas the server is usually a higher-powered multi-user system running very different database software designed to hold much more data and service many users.
One prior art approach to synchronization has been to transmit a copy of the replica from the server to each client. This approach is possible when all modifications are done at the server and propagated to the client for read-only use. When updates can also occur at the client, separate table pairs can be used to propagate changes in each direction.
Many organizations have, in the absence of any specific tools, developed adhoc solutions. These are time consuming to develop, difficult to maintain, non-general, and less reliable in that they generally do not properly recover from errors such as power failures, line failures, and system crashes.